Evolution of chromosome number in wild onions (Allium, Amaryllidaceae)

Polyploidy has been shown to be a significant driver of diversification among land plants.  In addition to whole-genome duplication, other common mechanisms of chromosome number evolution include increases by a multiple of 1.5 in chromosome number due to the fusion of gametes with different ploidy levels (demi-polyploidy), gains or losses of single chromosomes that alter the DNA content of an organism (aneuploidy), or chromosome fission or fusion (ascending dysploidy or descending dysploidy, respectively).  Considering the high variability in chromosome number transitions across multiple clades within angiosperms and the ancient genome duplication events responsible for their diversity, more studies of large polyploid systems are necessary to close the gaps in understanding chromosomal evolution in polyploid plants.  Allium (Amaryllidaceae) is an ideal candidate for polyploid research because it is a large clade that includes numerous natural populations of diploid and polyploid species.  Species of Allium mainly occupy temperate climates in the Northern Hemisphere and include economically important ornamentals and cultivated crops such as leeks, garlic, chives, and onions.  Here, we used a global molecular phylogeny of Allium to examine chromosomal evolution with chromEvol v. 2.0 which uses likelihood-based methods for inferring the pattern of chromosome number change across a phylogeny.  The best-fit model of chromosomal evolution indicated that chromosome transitions within Allium occurred through the constant gains and losses of single chromosomes as well as demi-polyploidization events, with the rate of chromosome gain events being approximately 2.5 to 4.5 times more likely to occur than demi-polyploidization and loss events, respectively.